Mean Bias (ppb): This performance statistic averages the difference (model - observed) over

all pairs in which the observed values were greater than zero. A mean bias of zero indicates that

the model over predictions and model under predictions exactly cancel each other out. Note that

the model bias is defined such that positive values indicate that the model prediction exceeds the

observation, whereas negative values indicate an underestimate of observations by the model.

This model performance estimate is used to make statements about the absolute or unnormalized

bias in the model simulation.

Normalized Mean Bias (percent): This statistic averages the difference (model - observed)

over the sum of observed values. Normalized mean bias is a useful model performance indicator

because it avoids over inflating the observed range of values.

Mean Fractional Bias (percent): Normalized bias can become very large when a minimum

threshold is not used. Fractional bias is used as a substitute. The fractional bias for cases with

factors of 2 under- and over-prediction are -67 and + 67 percent, respectively (as opposed to -50

and +100 percent, when using normalized bias). Fractional bias is a useful indicator because it

has the advantage of equally weighting positive and negative bias estimates. The single largest

disadvantage is that the predicted concentration is found in both the numerator and denominator.

Mean Error (ppb): This performance statistic averages the absolute value of the difference

(model - observed) over all pairs in which the observed values are greater than zero. It is similar

to mean bias except that the absolute value of the difference is used so that the error is always

positive.

n901 - n902 - n903 - n904 - n905 - n906 - n907 - n908 - n909 - n910 - n911 - n912 - n913 - n914 - n915 - n916 - n917 - n918 - n919 - n920 - n921 - n922 - n923 - n924 - n925 - n926 - n927 - n928 - n929 - n930 - n931 - n932 - n933 - n934 - n935 - n936 - n937 - n938 - n939 - n940 - n941 - n942 - n943 - n944 - n945 - n946 - n947 - n948 - n949 - n950

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